///pub mod apply {    /// Returned when failing to apply deltas.    #[derive(thiserror::Error, Debug)]    #[expect(missing_docs)]    pub enum Error {        #[error("Corrupt delta data: {message}")]        Corrupt { message: &'static str },        #[error("Encountered unsupported command code: 0")]        UnsupportedCommandCode,        #[error("Delta copy from base: byte slices must match")]        DeltaCopyBaseSliceMismatch,        #[error("Delta copy data: byte slices must match")]        DeltaCopyDataSliceMismatch,    }} /// Given the decompressed pack delta `d`, decode a size in bytes (either the base object size or the result object size)/// Equivalent to [this canonical git function](https://github.com/git/git/blob/311531c9de557d25ac087c1637818bd2aad6eb3a/delta.h#L89)pub(crate) fn decode_header_size(d: &[u8]) -> Result<(u64, usize), apply::Error> {    let mut shift = 0;    let mut size = 0u64;    let mut consumed = 0;    for cmd in d.iter() {        if shift >= u64::BITS {            return Err(apply::Error::Corrupt {                message: "delta header size uses more bits than fit into u64",            });        }        consumed += 1;        size |= (u64::from(*cmd) & 0x7f) << shift;        shift += 7;        if *cmd & 0x80 == 0 {            return Ok((size, consumed));        }    }    Err(apply::Error::Corrupt {        message: "delta header size is truncated",    })} pub(crate) fn apply(base: &[u8], mut target: &mut [u8], data: &[u8]) -> Result<(), apply::Error> {    fn next_byte(data: &[u8], i: &mut usize) -> Result<u8, apply::Error> {        let byte = *data.get(*i).ok_or(apply::Error::Corrupt {            message: "delta copy instruction is truncated",        })?;        *i += 1;        Ok(byte)    }     let mut i = 0;    while let Some(cmd) = data.get(i) {        i += 1;        match cmd {            cmd if cmd & 0b1000_0000 != 0 => {                let (mut ofs, mut size): (u32, u32) = (0, 0);                if cmd & 0b0000_0001 != 0 {                    ofs = u32::from(next_byte(data, &mut i)?);                }                if cmd & 0b0000_0010 != 0 {                    ofs |= u32::from(next_byte(data, &mut i)?) << 8;                }                if cmd & 0b0000_0100 != 0 {                    ofs |= u32::from(next_byte(data, &mut i)?) << 16;                }                if cmd & 0b0000_1000 != 0 {                    ofs |= u32::from(next_byte(data, &mut i)?) << 24;                }                if cmd & 0b0001_0000 != 0 {                    size = u32::from(next_byte(data, &mut i)?);                }                if cmd & 0b0010_0000 != 0 {                    size |= u32::from(next_byte(data, &mut i)?) << 8;                }                if cmd & 0b0100_0000 != 0 {                    size |= u32::from(next_byte(data, &mut i)?) << 16;                }                if size == 0 {                    size = 0x10000; // 65536                }                let ofs = ofs as usize;                let end = ofs.checked_add(size as usize).ok_or(apply::Error::Corrupt {                    message: "delta copy range overflows",                })?;                std::io::Write::write(                    &mut target,                    base.get(ofs..end).ok_or(apply::Error::Corrupt {                        message: "delta copy range exceeds base object size",                    })?,                )                .map_err(|_e| apply::Error::DeltaCopyBaseSliceMismatch)?;            }            0 => {                return Err(apply::Error::Corrupt {                    message: "delta command 0 is reserved and invalid",                });            }            size => {                let end = i.checked_add(*size as usize).ok_or(apply::Error::Corrupt {                    message: "delta insert range overflows",                })?;                std::io::Write::write(                    &mut target,                    data.get(i..end).ok_or(apply::Error::Corrupt {                        message: "delta insert data is truncated",                    })?,                )                .map_err(|_e| apply::Error::DeltaCopyDataSliceMismatch)?;                i = end;            }        }    }    debug_assert_eq!(        i,        data.len(),        "delta instructions were not consumed completely, should be impossible"    );    if !target.is_empty() {        return Err(apply::Error::Corrupt {            message: "delta instructions produced fewer bytes than promised",        });    }     Ok(())}